2J.S 



KN()\VLl-,l)C.i:. 



Jink. 191.'. 



rapid aiul siiililni. tlioiiKli less pcriiianciil. And it is furthur 

 conceivable lliat chaiiKcs due Id factors of intermediate orders 

 iiiiclit occur : so that essenliallv Darwin and his later critics 



''"■'■''■■''■''•■■'•■ I ' ■uiniilati'd a i ' ' t>ser- 





•IGUKli 264. 

 Kadula of Zoniioi scharfjl Keiiiiard. In paralliii. 



tions upon these points, starting with the snails' tongues as 

 text; since circumstances do not permit me to pursue the 

 inatter at present, it seems well to thus lay the skeleton of my 

 conclusions before the readers of " Knowledge." There is 

 another very interesting point that is brought out by the com- 

 parative study of radulac. There are strong indications that 

 the inter -relationship of all these molluscs is in reality much 

 closer than systeniatists would be inclined to allow. Science 

 is description, and much of it has become very minute 

 description ; but it may be that in the near future more energy 

 will be expended on the study of possible evolutionary factors 

 than on the careful search for (une.\plainedt specific diflfer- 

 ences, or on the construction of elaborately artificial classifica- 

 tions. These remarks may tend to show that the snail's 

 tongue offers a fertile field for investigation to the microscopist 



who is slightly predisposed in favour of beautiful objects, and 

 at the same time has a preference for a line of study which 

 may have a direct bearing upon questions of scientific 

 philosophy. The object of this note is to point out the interest 

 of the subject, and to describe a method of making permanent 

 preparations which- is the result of many years of experiment. 

 There are, of course, other microscopical objects to which the 

 same remarks will in a measure apply. 



The first practical (|ucstion to decide is whether the struc- 

 tural details are to be rendered visible by diflfercnces of 

 refraction, or by dissimilar absorption ; in other words, whether 

 we are to de.il with them as stained or as unstained objects. 

 The suggestion is often made that these two methods should 

 be combined, so that one has a stained object mounted in a 

 mcditnn of refractive index higher or lower than its own. Let 

 a radul.i of I.iiiiiuicii sliifinalis be stained with carbol-fiichsin 

 and mounted in monobromide slyrax ; or let an ordinary stained 

 section be mounted in some resinous medium of lower refrac- 

 Uvc index than Canada balsam. The result is that one has a 

 preparation that can be examined as a stained object, and also 

 .IS an unstained one ; and if in any case it is important to be able 

 to change from one style to the other by the simple means of 

 opening and shutting the iris of the condenser, this combination 

 may be desirable : but it must be added that the specimen 

 which has these double properties is not so good for either 

 purpose singly as the specimen specially mounted for 

 observation in one or other of the two ways. It may be 

 noticed here that if a specimen is to be examined with 

 polari.sed light or by dark-ground illumination, the visibility of 

 its details may generally be augmented by staining it with a 

 fluorescent stain. This looks as if the stain was merely 

 adsorbed, and had no chemical reaction with the tissue. 



Kadula of Siicciiica elcgaiis Kisso. In paraffin. 

 Zeiss dark-ground condenser. 



With objects mounted for refraction contrast, the actual 

 numerical aperture has a distinct influence on the formation of 

 the image by contrast of refraction, according to the angle of 

 delivery of the rays. A radula in paraffin, for example, may 

 l>e clearly seen as a black object with a \ery low power, 

 having condenser and objective equally balanced : contrast 

 sufficient to show details may be evident when this is 

 <xchanged for an objective of N.A. -20. the condenser being 

 opened to match : exchanging further for an objective of K..A. 

 • JO, and again matching, the outlines seem incHned to 

 disappear, though what you do see is very sharp : but when you 

 i;et to N.A. -85 it may be impossible to see anything by 

 contrast without shutting down the condenser iris. However. 

 I lie phenomenon does not stop there, for if now you exchange 

 for an oil immersion of N..-\. 1-30, you will again see the 

 contrast image, without any shutting down of the condenser 

 aperture, this being made equal to that of the objective by 

 the usual means. On the other hand, the improvement in 

 definition obtained by using objectives of higher numerical 

 aperture on stained objects in a homogeneous medium, is 

 progressive and continuous, the entire aperture being used. 

 Here, obviously, it is not possible to improve definition by any 

 closing of the iris ; where this plan appears to give improve- 

 ment, it is due either to contrast effect obtained between two 

 elements of the preparation which do not happen to be of 

 exactly the same refracti\e index, or to a deficiency in the 



